1. Field of the Invention
The present invention relates to a voltage regulator and in particular to an output stage of a voltage regulator that can increase its current without adverse impact on circuit operation, cost, or battery life.
2. Related Art
Portable electronic devices, such as laptops and wireless communication devices, have increasingly sophisticated functionality with longer battery life and/or longer time between charges. A low dropout (LDO) regulator is frequently used in such portable electronic devices. In general, a voltage regulator can reduce an input voltage, thereby providing a regulated output voltage. LDO regulators can advantageously provide a significantly smaller minimum required voltage between the input/output voltages, i.e. the dropout voltage, than standard voltage regulators. The dropout voltage has a direct relationship to the battery life of the device. Specifically, the smaller the dropout voltage of the LDO regulator, the longer the battery life.
FIG. 1A illustrates a conventional LDO regulator 100. In LDO regulator 100, an error amplifier 101 provides its output to a gate buffer 109, which in turns provides its output to the gate of a pass (PMOS) transistor 102. Pass transistor 102 has its source connected to voltage source VDD (also called Vin) and its drain connected to a node 103 that provides voltage VOUT (which drives a load 107). Note that although shown schematically as a single transistor, pass transistor 102 typically includes many transistors, e.g. on the order of thousands of transistors, and therefore is also called a “power device” in the industry. Resistors 104 and 106 are connected in series between node 103 and a voltage source VSS. A node 105, which is located between resistors 104 and 106, provides a feedback voltage to the positive input terminal of error amplifier 101. A reference voltage Vref, which is typically generated by a bandgap circuit, is provided to the negative input terminal of error amplifier 101. In this configuration, pass transistor (hereinafter, power device) 102 can provide a relatively low dropout voltage, e.g. 60 mV compared to 2 V for standard regulators.
FIG. 1B illustrates an improved LDO regulator 110 that can improve gate drive without increasing input voltage or device size. Specifically, improved LDO regulator 110 can forward bias the body to source junction of power device 102. This forward biasing can advantageously reduce the threshold voltage of power device 102, thereby effectively increasing its gate drive as well as its output current capability. In other words, for the same gate bias, more current can flow through power device 102 when it is forward biased.
Notably, the forward biased junction of power device 102 is defined by the voltage drop across Schottky diode 111, which has its anode connected to voltage source VDD. In LDO regulator 110, a PMOS transistor 112 and NMOS transistors 113 and 114 can form a bias circuit that limits the current through Schottky diode 111. In this embodiment, the gate of PMOS transistor 112 can receive an output of error amplifier 101, its source can be connected to voltage source VDD, and its body can be forward biased. The drain of PMOS transistor 112 can be connected to the drain and gate of NMOS transistor 113. The sources of NMOS transistors 113 and 114 are connected to voltage source VSS, the gate of NMOS transistor 114 is connected to the gate of NMOS transistor 113, and the drain of NMOS transistor 114 is connected to the cathode of Schottky diode 111. Because NMOS transistors 113 and 114 form a current mirror in this configuration, the current through PMOS transistor 112 can determine the current through Schottky diode 111 by controlling its forward bias.
Note that PMOS transistor 112 has a defined relationship to power device 102, i.e. the sizing and construction of PMOS transistor 112 is substantially identical to a constituent transistor of power device 102. Therefore, a current through PMOS transistor 112 should be substantially proportional to the current through power device 102.
Unfortunately, an actual implementation of LDO regulator 110 has significant disadvantages. Specifically, Schottky diodes are infrequently used in the industry and therefore undesirably increase the cost of the implemented circuits. Moreover, even if available, Schottky diodes have a high leakage current, e.g. increasing ground current by as much as 360%. A high leakage current can significantly reduce battery life in portable applications. Therefore, LDO regulator 110 including Schottky diode 111 would not be a commercially viable implementation.
Therefore, a need arises for an output stage of a voltage regulator that can increase the current of the power device without adverse impact on circuit operation, cost, or battery life.